GEOMORPHIC BASED LANDSLIDE HAZARD MAPPING, NAM CHUNG TSUEN, HONG KONG SAR, CHINA

Intense rainfall on June 7, 2008 triggered about 1600 natural terrain landslides on Lantau Island, Hong Kong, 60 of which occurred near the villages in the coastal Tai O valley in west Lantau Island. Road closures resulting from the landsliding left the communities isolated for up to two weeks. Emergency landslide mapping was performed by ferrying in geologists by boat to assess immediate landslide hazards and assist in evaluating potential evacuations. Seven landslides, including three relatively large channelized debris flows and one large debris avalanche, occurred on the natural hillslopes and channels above Nam Chung Tsuen and neighboring small villages, threatening residences and resulting in the permanent evacuation of 8 structures and temporary evacuation of 22 structures.

Detailed geomorphic mapping of the recent landslide trails was inhibited by emergency engineering measures that used shotcrete to cover the debris flow paths in an attempt to control erosion. Therefore various forensic techniques were used to supplement field mapping including evaluation of aerial and ground photographs taken immediately after the landsliding. Geomorphic mapping of the unaffected hillslopes helped to assess the magnitude and history of landsliding in the area and allow for estimation of potential failure geometries and debris volumes.

Mapping of the recent landslide source areas identified failure mechanisms that included sliding along open, adversely oriented joint sets that were infiltrated by concentrated surface water at the base of cliffs. Channelization of debris from several of the landslides resulted in on-slope run-out lengths ranging from 250 m to 470 m. Entrainment of debris from the channel sides and bottoms below the source areas resulted in up to a 9 fold increase in the total debris flow volume.

Geomorphic mapping of the catchment above Nam Chung Tsuen revealed landsliding as the dominant hillslope process in the study area and facilitated assessment of previous landslide source areas, geometries, and volumes. Landslide processes as well as the relative age and potential recurrence level were also evaluated for the region through geomorphic analyses. These derivative products were critical to the design of mitigation works implemented to protect the down-slope communities from future landslide hazards.